System unit for desorbing carbon dioxide from methanol

ABSTRACT

The invention relates to a system unit for desorbing carbon dioxide and other impurities from highly pressurized methanol. Said system unit comprises at least one or more expansion vessels arranged in succession, at least one heat exchanger, and at least one liquid/gas separator. The inventive system unit contains: a) a line ( 1 ) through which the intensely cooled methanol leaving expansion vessel (C) is introduced into the heat exchanger (E) from underneath, and; b) a line ( 2 ), though which the heated methanol is drawn out of the heat exchanger (E) from the top, and which serves to connect said heat exchanger to a liquid/gas separator inside of which the remaining carbon dioxide still contained in the methanol is desorbed and separated out to the greatest possible extent. This system unit enables the cold due to evaporation, said cold resulting during the desorption of carbon dioxide, to be obtained inside a heat exchanger and constitutes an important cold energy source for carrying out absorption.

The object of the invention is a system unit, which in the totalpurification of compressed gases, makes it possible to recover methanolin a pure state and at the same time utilize the cold energy liberatedin an extremely effective way.

It is known that cold methanol has the capability of absorbing largeamounts of gas impurities. Use is made of this capability in theRectisol® process, in which the total purification of compressed gasesis possible in a single process operation. The absorptive capability ofmethanol increases considerably at lower temperatures. At −60° C. forexample 75 times more carbon dioxide dissolves in methanol than in thesame volume of water at 25° C., that is the methanol cycle amounts to1/75 compared with water recirculation in a pressurized water cycle. Atlower temperature the vapor pressure of methanol is so low that there islittle use of solvent.

The Rectisol® process is particularly efficient when large amounts ofgas impurities must be removed, or if a particularly high gas purity isrequired, and overall where the process can be built into the cold stageof a low temperature gas decomposition. In the latter case the processitself offers important advantages when only small quantities of gaseousimpurities are to be washed out.

Gas washing is carried out such that the standing raw gas is treatedwith methanol under medium pressure of 5 to 40 atmospheres or also underhigh pressure of 50 to 200 atmospheres at temperatures between 10° C.and −80° C.

Thereby, all gaseous impurities such as raw gasoline, crude benzene,ammonia, hydrocyanic acid, resin formers, organic sulfur and phosphoruscompounds, carbonic acid, hydrogen sulfide, iron and other metalcarbonyls and water are absorbed. The loaded up methanol is thenregenerated by expansion, evaporation, or heating, and subsequentlyre-used. The impurities can be recovered from the off-gases orcondensate. This process is the subject of German patent 1 544 080.

An especially important gas purification process known to the artincludes purification of, for example, synthesis gases, produced fromnatural gas gasification processes, which are the starting point for avariety of major technical syntheses. Raw synthesis gas containsconsiderable amounts of carbon dioxide the removal of which is cruciallyimportant for the further use of synthesis gasses. The development ofeffective, reliable, and cost effective processes for removal of carbondioxide from synthesis gasses is therefore of considerable importancefor the efficient winning of a variety of applicable gas mixtures.

It has now been found that the process known up until now for thepurification of gases with methanol may still be improved considerably,if the system unit in accordance with the invention, and the processthat may be carried out therein for desorption of carbon dioxide, isemployed.

The object of the invention is therefore a system unit for desorption ofcarbon dioxide and other impurities from methanol held under highpressure, comprising one or a plurality of expansion vessels arranged insequence, at least one heat exchanger and at least one liquid/gasseparator, in which

-   (a) A line (1) is provided through which the strongly cooled    methanol leaving the expansion vessel C is introduced from below    into the heat exchanger and-   (b) a line (2) is provided, through which the heated methanol above    is transported from the top of the heat exchanger E, and is    connected to a liquid/gas separator, in which the remaining carbon    dioxide contained in the methanol is desorbed and separated to the    greatest extent possible.

FIG. 1 shows an entire installation for desorption of carbon dioxide andother impurities from methanol held at high pressure, while FIG. 2depicts the system unit in accordance with the invention and disclosesfurther technical details thereby.

For effective implementation of the process in accordance with theinvention it is of great importance that the three reaction vessels C, Dand E be arranged at a carefully determined height relative to eachother. That allows one in fact to ensure that the liquid flows in thewanted direction through the heat exchanger E, without a pump beingnecessary. Liquid flow known as the thermo-siphon effect developsautomatically as a result of gravity and the condensing carbon dioxide.That can only be achieved though in the system unit in accordance withthe invention

-   (a) the liquid level in the expansion vessel C is located about 1 to    20 m above the liquid level in the liquid-gas separator;-   (b) this again is located about 0.5 m above the top of discharge    opening for the heated methanol provided in the heat exchanger E;-   (c) the distance between the inlet line (1), from the bottom of the    heat exchanger E, for the methanol fed from the expansion vessel C,    and the base of the heat exchanger E is about 0.5 m.

Self-evidently the system unit in accordance with the invention can beoperated by use of pumps, however it is particularly advantageous toexploit application of the thermo-siphon effect so to automaticallyestablish liquid flow through the various components of the system unitin accordance with the invention.

The system unit in accordance with the invention is downstream to anabsorber (5), which is provided for purification of synthesis gas withmethanol. In addition, in accordance with the invention a regenerator(6) is downstream to the system unit, in which, by further increasingthe temperature and influx of a heated inert gas—such as for examplemethanol vapor—the remaining carbon dioxide is desorbed from themethanol.

In the absorber (5) the raw gas flowing in from below through thecounter-flowing cold methanol, is purified. The outgoing liquid frombelow the absorber (5), containing all impurities of the raw gas, iscooled in the heat exchanger E and fed into the expansion vessel (A).The purified synthesis gas leaves at the top of the absorber.

In the expansion vessel A the methanol held under a pressure of 55atmospheres is expanded to about 9 atmospheres and at a temperature of−45° C. desorbs mainly hydrogen and carbon monoxide, which after passagethrough the heat exchanger E are obtained as gas fraction for theprocess. The liquid fraction from the expansion vessel A is then fedthrough a line to a second expansion vessel B.

In expansion vessel B the methanol pressure is lowered from about 9atmospheres to about 2.7 atmospheres and thereby a temperature decreasefrom about −45° C. to about −52° C. is obtained. In this case gaseouscarbon dioxide is released from the methanol, which is passed throughthe heat exchanger E and may subsequently be credited to the process,while the liquid fraction obtained is fed to a third expansion vessel C.

In expansion vessel C the pressure of the methanol solution is decreasedfrom about 2.7 atmospheres to about 1.2 atmospheres and thereby afurther temperature decrease from about −52° C. to about −60° C. isobserved. Also in this expansion vessel, gaseous carbon dioxide isobtained, which likewise is fed to heat exchanger E and can subsequentlybe credited to the process.

The liquid fraction obtained in expansion vessel C is then preferablydivided into two streams, wherein one stream is fed to the upstreamabsorber (5) and the second stream is fed through line 1 to the heatexchanger E, which in its case, for the methanol heated there, isconnected by line (2) with the liquid-gas separator D.

The liquid-gas separator D has a branch line (3) for gaseous carbondioxide, as well as another line in which the liquid methanol is takenfrom below the separator and fed to the downstream regenerator (6). Theliquid fraction (4) taken from the liquid/gas separator is fed to thedownstream regenerator (6) to remove the last traces of carbon dioxide,which are extracted by further increasing the temperature and feeding ina stream of heated gas, for example methanol vapor. While the carbondioxide is taken from the process, the ultrapure methanol produced inthe regenerator is fed back to the absorber (5) and remains there to beavailable again for the purification of a fresh stream of raw gas.

Overall the process in accordance with the invention is thereforecharacterized in that carbon dioxide is desorbed from methanol stepwisein a plurality of expansion vessels, at least one heat exchanger, and atleast one liquid/gas separator. Here the methanol leaving the expansionvessel has a temperature of −60±10° C. and a pressure of 1 to 2atmospheres. The cold liberated by the heat exchanger E represents avaluable energy source available for other cooling reactions. In thisinstance the temperature of the methanol stream increases in the heatexchanger to −10±5° C., and the liquid stream is fed to the liquid/gasseparator at this temperature.

The process in accordance with the invention and the system unitassociated with it thus make possible in an exceptionally purposefulmanner the purification of enriched methanol, in the total removal ofthe contained pressurized gases and impurities, especially carbondioxide. At the same time, the cold of vaporization resulting fromdesorption of carbon dioxide is recovered which is of great significancefor absorption processing.

The material streams in the system unit in accordance with the inventionshow the indicated parameters in table 1 below. Material Stream StreamParameters 1 2 3 4 Carbon Dioxide 11.45 11.45 98.10 1.44 Methanol 88.5588.55 1.92 98.56 Temperature −59.5 −8.8 −8.9 −8.9 Pressure (in absolute1.20 1.20 1.15 1.15 atmospheres) Vapor Proportion 0.00 0.10 1.00 0.00Flow Velocity (t/h) 585 585 80 505

1. A system unit for desorption of carbon dioxide and other impuritiesfrom high pressure methanol comprising one or a plurality ofsequentially arranged expansion vessels, at least one heat exchanger,and at least one liquid/gas separator, characterized in that (a) a line(1) is provided through which the intensely cooled methanol leaving theexpansion vessel C is fed from below a into a heat exchanger E; and (b)a line (2) is provided through which the heated methanol is fed fromabove the heat exchanger E and is connected to a liquid/gas separator,in which the remaining carbon dioxide still contained in the methanol isdesorbed to the greatest extent possible; (c) wherein the liquid levelin the expansion vessel C is located about 1 to 20 m above the liquidlevel in the liquid/gas separator D; and (d) wherein the liquid level inthe liquid/gas separator D is located about 0.5 m above the exit openingprovided for heated methanol in the top of the heat exchanger E.
 2. Thesystem unit according to claims 1, characterized in that it isdownstream to an absorber (5), which is provided for purification ofsynthesis gas with methanol.
 3. The system unit according to claim 1,characterized in that a regenerator (6) is downstream to it, in which byfurther increasing the temperature and influx of heated inert gas theremaining carbon dioxide is desorbed from the methanol.
 4. The systemunit according to claim 1, characterized in that the first expansionvessel A for the gas mixture obtained by desorption comprising hydrogenand carbon monoxide, has a line going to the heat exchanger E and a lineto the expansion vessel B for the methanol containing liquid.
 5. Thesystem unit according to claim 1, characterized in that the secondexpansion vessel B for the carbon dioxide gas obtained by desorption hasa line going to the heat exchanger E and a line to the expansion vesselC for the methanol containing liquid.
 6. The system unit according toclaim 1, characterized in that the expansion vessel C for the gaseouscarbon dioxide obtained by desorption has a line (1) going to the heatexchanger E and a line for the methanol containing liquid to theupstream absorber which for its part is connected by line (2) themethanol heated up there to the liquid/gas separator D.
 7. The systemunit according to claim 1, characterized in that the liquid/gasseparator D has a branch line (3) for the gaseous carbon dioxide andanother line (4) provided for feeding the separated methanol to thedownstream regenerator.
 8. The process for desorption of carbon dioxideand other gaseous impurities from methanol in the system unit inaccordance with claim 1, wherein the desorption is carried out stepwisein a multiplicity of sequentially arranged expansion vessels, at leastone heat exchanger and at least one liquid/gas separator, characterizedin that the methanol leaving the expansion vessel C at a temperature of−60±10° C. and a pressure of 1 to 2 bar is fed into the heat exchangerE, heated there to a temperature of −10±5° C. and fed into theliquid/gas separator D.
 9. The process according to claim 8,characterized in that the further material flow between the expansionvessels A, B and C as well as to the heat exchanger E and to theliquid/gas separator D may be accomplished with the aid of pumps orpreferably by utilization of the thermo-siphon effect.
 10. The processaccording to claim 8, characterized in that in the expansion vessel Athe pressure decreases from about 55 bar to about 9 bar and mainlyhydrogen and carbon monoxide are desorbed at a temperature of about −45°C., wherein the gas fraction obtained after passing through the heatexchanger E is recovered to the process, while the liquid fraction isfed to a second expansion vessel B.
 11. The process according to claim8, characterized in that in the second expansion vessel B the pressuredecreases from about 9 bar to about 2.7 bar and gaseous carbon dioxideis obtained at a temperature of about −45° C., to about −52° C., whichis fed through the heat exchanger E and subsequently obtained for theprocess, while the liquid fraction obtained is fed to the thirdexpansion vessel C.
 12. The process according to claim 8, characterizedin that, in the third expansion vessel C, the pressure of about 2.7 bardecreases to about 1.2 bar and gaseous carbon dioxide is obtained at atemperature of about −52° C., to about −60° C., which is fed through theheat exchanger E and subsequently can be obtained for the process. 13.The process according to claim 8, characterized in that the liquidfraction contained in the third expansion vessel C is divided into twostreams wherein one stream is fed to the upstream absorber (5) and thesecond stream after passing through the heat exchanger E via line (2) isfed to the liquid/gas absorber D.
 14. The process according to claim 8,characterized in that the liquid fraction (4) recovered in theliquid/gas separator D is fed to a downstream regenerator (6) forremoval of the last traces of carbon dioxide and the gas fraction (3)preferably purified with further carbon dioxide rich gas fractions isobtained to the process.